Primary preforms are used to make optical fibers, and related processes for manufacturing primary preforms are known.
For example, U.S. Pat. No. 5,970,083, which is hereby incorporated by reference in its entirety, discloses a collapsing device comprising a graphite furnace that surrounds a cylindrical envelope, with a carrier tube being movable within the cylindrical envelope in the longitudinal direction thereof. The device is used for collapsing a hollow rod approximately 30 millimeters in diameter, which must be transformed into a preform that can be used for drawing the optical fiber (i.e., a solid rod having a diameter of approximately 20 millimeters). This transformation occurs principally towards the middle of the furnace (a hot zone), where a temperature of approximately 2000° C. prevails. The cylindrical envelope comprises an inlet opening and an outlet opening, both openings including two rings of conduits through which a non-oxidizing gas is passed. The annular conduits are inclined at an angle to the axial direction of the envelope, and the non-oxidizing gas is injected in two conical gas curtains by the annular conduits. The gas that is thus injected is directed away from the envelope, such that any entry into the envelope of air that could cause combustion of the graphite furnace is prevented.
A disadvantage of this method is that impurities that are present on the outside of the deposited tube (e.g., in the form of soot) and/or that are introduced during the collapsing process are incorporated into the outer glass layers of the primary preform during the collapsing step. Soot may also form bubbles during the collapsing process, which is undesirable. Therefore, there is a need for an improved collapsing method that provides primary preforms having a reduced contamination content.